In a third revision, a productive, senior investigator requests five years of support to study cGMP in the turtle retina. The proposal has three specific aims: (1) to test the hypothesis that ganglion cell activity can modulate levels of cGMP in the retina: this aim will be accomplished by stimulation of the optic nerve to test the hypothesis that NO can function as a retrograde transmitter; (2) to localize isoforms of NOS and investigate which specific retinal cell types show changes in cGMP activity after stimulation of glutamate receptors, cholinergic receptors, ischemia and other manipulations of cGMP-related enzymes: this aim is designed to indicate which cells and pathways in the retina are influenced by cGMP mechanisms; and (3) electron microscopic analysis of NOS neurons and those neurons which show changes in cGMP levels in response to atrial natriuretic factors: this aim will provide information on the synaptic control of cGMP mechanisms.